The reproductive toxicity and carcinogenicity of new as well as clinically useful antitumor agents are under investigation. Studies on the embryotoxic effects in mice of hexamethylmelamine, pentamethylmelamine and demethylated analogs have been completed. The effect of long-term, low dose treatment with cyclophosphamide or procarbazine on fertility in male mice and on malformations, survival and tumor incidence in their offspring has been evaluated. The potential carcinogenic activity of various clinically effective antitumor and/or immunosuppressive agents, including procarbazine, N-methylnitrosourea, melphalan, azathioprine, adriamycin and cyclophosphamide, is under evaluation in long-term studies in monkeys. A survey of second malignancies developing in treated cancer patients and on the effects of these agents on the fetus, initiated several years ago, is continuously being updated using data from published reports and from centers and hospitals.